The preset average or 'house' drilling is only recommended for those getting their first ball or have no access to a pro shop. These drillings are available as a drop down option on polyester balls, novelty balls and a few of the starter reactive urethanes.  As house balls hole sizes are not constant from bowl to bowl, below is an approximate guide to the hole diameter sizes and spans drilled by Bowling World 2000:
XS - Thumb 17mm (21/31") , Fingers 14mm (9/16"),  Span 50mm (2")

S - Thumb 17.5mm (11/16), Fingers 16mm (5/8"), Span 64mm (2 1/2")

SM - Thumb 19mm (3/4"), Fingers 17mm (21/32"), Span 70mm (2 3/4")

M - Thumb 20.5mm (13/16"), Fingers 17.5mm (11/16") , Span 76mm (3")

M/L - Thumb 22mm  (7/8"), Fingers 19mm (3/4"),  Span 80mm, (3 1/8")

L - Thumb 24.5mm (31/32") , Fingers 20.5mm (13/16"), Span 83mm (3 1/4")

XL - Thumb 25.4mm (1"), Fingers 21.5mm (27/32"),  Span 86mm (3 3/8")

XXL - Thumb 26.5mm  (1 1/32"),  Fingers 23mm (29/32"),  Span 89mm (3 1/2")

As the thumb is the most important of the above measurements the following size comparisons may be helpful. Please note this is a guide only and it is still the customers responsiblity to ensure the measurements you choose are right for you.

17mm (21/32") is approximately ring gauge size M.

17.5mm (11/16") is approximately ring gauge size P.
19mm (3/4") is approximately ring gauge size S.
20.5mm (13/16") is approximately ring gauge size W.

22mm (7/8") is very slightly larger than the largest ring gauge size Z+1/2.

24mm (31/32") is the inside diameter of a Gaviscon Tube.

26mm (1 1/64") is the inside diameter of most small rolls of Scotch Tape that fit in a dispenser.

There are ways of measuring your thumb size which can be viewed under Measuring Your Hand which is also on this horizontal menu.

Please note that balls drilled to the above measurements cannot be returned unless there is a fault with the ball.

All measurements are nominal.